The applicant has previously proposed, for instance in Japanese patent laid-open publication No. 8-21492 which corresponds to the U.S. patent application Ser. No. 08/497,557 filed Jun. 30, 1995, a torque splitting device which, provided in parallel with a conventional differential device, controls the simulated rolling resistance to each of the right and left or front and rear axles and boosts the rotational speed of the axle encountering a lower rolling resistance. Thereby, the torque distribution ratio to the right and left axles can be positively changed depending on the steering wheel steering angle and the vehicle speed to the end of improving the steering performance of the vehicle. The contents of the above mentioned United States patent application are hereby incorporated in this application by reference.
As illustrated in FIG. 9, this previously proposed torque splitting device T comprises an oil pressure pump 32 producing an output pressure that depends on the vehicle speed, a regulator Re for adjusting the output pressure to a prescribed level, a pair of wet hydraulic multi-disk clutches Ca and Cd for producing simulated rolling resistances, a pressure regulating valve 30 consisting of a linear solenoid valve for determining a torque distribution ratio for the right and left (or front and rear) wheels according to the turning radius or the road resistance, and controlling the engagement forces of the clutches Ca and Cd so as to achieve a desired torque distribution ratio by adjusting the oil pressure for each of the clutches to a target value, an electronic control unit 29 for computing the target oil pressures, and controlling the electric current for the pressure regulating valve 30, and a planetary gear mechanism P which is connected to the wet hydraulic multi-disk clutches Ca and Cd and actually distributes the torque. The output of the engine E forwarded to the torque splitting device T via the transmission TM can be thus appropriately distributed to the right and left (or front and rear) axles 5L and 5R depending on the operating condition of the vehicle.
Thus, by appropriately distributing the engine drive torque to the right and left driven wheels, it is possible to produce a yaw moment which assist the vehicle to go into a turn while involving a relatively small side slip angle of the front wheels. However, when a front wheel drive vehicle does a relatively tight turn while applying an engine brake to the driven wheels or the front wheels, because the side slip angle of the front wheels increases, a moment is produced which tends to turn the vehicle inward with respect to the turning circle, or an oversteer tendency, is produced as is well known in the art. Conversely, when a front drive vehicle accelerates while turning, the vehicle tends to demonstrate an understeer tendency.
However, when the vehicle is equipped with a torque splitting device, it is possible to eliminate such a tendency by appropriately controlling the torque distribution to the right and left front wheels. This is generally advantageous because most vehicle operators do not particularly prefer the vehicle to show different turning behaviors depending on if the vehicle is accelerating, decelerating, or traveling at constant speed.
However, there are also those vehicle operators who prefer to retain the behavior of the conventional front drive vehicle when turning. In particular, the vehicle operator may rely on the oversteer tendency of the vehicle when going into a tight turn under an engine brake condition or, in some cases, may even deliberately release the accelerator pedal and produce an engine brake condition to force the vehicle to make a sharper turn than otherwise possible.